AS the death toll in Syria has climbed to perhaps 7,000, proponents of humanitarian intervention
are asking, quite reasonably, why the West does not intervene as it did in Libya last year. Not only was Libya’s dictator, Col. Muammar el-Qaddafi, ousted with relatively few Western casualties, but the NATO campaign also set a precedent for successful humanitarian intervention.

In the 63 years since the United Nations adopted a genocide convention in the wake of the Holocaust, world leaders have failed to prevent the deaths of millions, from Biafra and Cambodia to Rwanda and Darfur — not just because they have lacked the political will to intervene, but also because of the norm of genocide itself. By setting the bar for intervention so high — unmistakable evidence of clear intent to destroy a national, ethnic, racial or religious group — the international community has stuck itself in a Catch-22: by the time it is clear that genocide is occurring, it is often too late to stop it.

A new standard for humanitarian intervention is needed. If a continuing government-sponsored campaign of mass homicide — in which thousands have died and many thousands more are likely to die — is occurring, a coalition of countries, sanctioned by major international and regional institutions, should intervene to stop it, as long as they have a viable plan, with minimal risk of casualties for the interveners.

The recent war in Libya was a case in point. When large parts of Libya broke away from Colonel Qaddafi’s rule, he retaliated with tanks, air power and artillery against heavily populated urban areas. His loyalists promised “rivers of blood.” The signs of impending state-sponsored mass murder were clear.

For weeks, the United States and other nations appeared paralyzed, unclear whether Colonel Qaddafi’s brutality would reach the level of genocide, while Robert M. Gates, then the defense secretary, fretted about the open-ended costs in the “ouster of a Middle Eastern leader” and the fallout from attacking “yet another Muslim country.”

But rather than seeking regime change to prevent genocide, President Obama focused on the narrower objective of preventing “a humanitarian catastrophe” and explicitly ruled out foreign-imposed regime change.

These more modest, pragmatic goals sidestepped Mr. Gates’s objections and reflect the emerging new standard for humanitarian intervention. The United States took the lead, but initially only to halt the mass-homicide campaign. And it rightly set goals that would not require an ambitious military commitment.

Libya was a success — and it was as low-risk as any United States military mission of the past 20 years. Colonel Qaddafi’s threat to civilians rested on his ability to direct heavy concentrations of weapons against rebel-controlled populated areas and to cut off supplies into ports; NATO airpower could blunt both tactics.

Within weeks, the threat to eastern Libya was minimized, giving the rebel movement breathing space to gain cohesion and battlefield experience and eventually defeat Colonel Qaddafi’s small and increasingly unpopular army.

In the past few decades, the United States and other countries have successfully intervened for humanitarian purposes on three other occasions — in 1991, to stop Saddam Hussein’s attempted massacre of the Kurds in northern Iraq after the gulf war, and to protect first Bosnians, in 1993, and then Kosovars, in 1999, from the Serbs’ attempts at ethnic cleansing. All three humanitarian interventions occurred after thousands of people had been killed and exponentially more people had been injured or displaced. And all three were successful and saved thousands of lives.

None of these cases, nor the war in Libya, amounted to true genocide, where hundreds of thousands were already dead at the time of intervention. Most important, none could become a genocide because intervention stopped the killing at an earlier stage.

Limited military force to stop campaigns of state-sanctioned homicide is more pragmatic than waiting for irrefutable evidence of “genocide.” It will not work in every case, but it will save large numbers of lives. It also promotes restraint in cases where humanitarian intervention would be high-risk or used as a pretext for imperial designs.

As the world’s sole military superpower, the United States will be at the center of many future debates over humanitarian action. Rather than hewing to the old standard of intervening only after genocide has been proved, the emerging new standard would allow for meaningful and low-risk military action before the killing gets out of control.

Syria is, I admit, a tough case. It is a borderline example of a government’s engaging in mass killings of its citizens. The main obstacle to intervention is the absence of a viable, low-casualty military solution. Unlike Libya, where much of the coastal core of the population lived under rebel control, the opposition to Syria’s dictatorial president, Bashar al-Assad, has not achieved sustained control of any major population area. So air power alone would probably not be sufficient to blunt the Assad loyalists entrenched in cities, and a heavy ground campaign would probably face stiff and bloody resistance.

If a large region broke away from the regime en masse, international humanitarian intervention could well become viable. Until then, sadly, Syria is not another Libya. A mass-homicide campaign is under way there, but a means to stop it without unacceptable loss of life is not yet available.

Robert A. Pape is a professor of political science at the University of Chicago.